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      synchronized 的锁升级机制 
      
      
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      <div class="post-title">synchronized 的锁升级机制</div>
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          2021-09-16 16:54:39
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        <p><img src="http://qiniu.c77544s.top/picgo/202210121457816.png" alt="synchronized锁升级流程图"></p>
<p>Synchronized关键字可以用来修饰方法或者代码块。对于同步方法，JVM 采用 ACC_SYNCHRONIZED 标记符来实现同步。 对于同步代码块。JVM 采用 monitorenter、monitorexit 两个指令来实现同步。</p>
<p>在JDK1.6之后对对synchronized锁进行了升级，在1.6之前synchronized直接加重量级锁，但是1.6之后对其性能进行了优化，采用锁升级的机制：偏向锁-&gt;自旋锁(轻量级锁)-&gt;重量级锁。</p>
<p>初次执行到synchronized代码块的时候，锁对象变成偏向锁。线程执行并不会主动释放锁。第二次到达代码块的时候，线程会判断之前的线程是不是自己，如果是自己的话，由于前面没有释放锁的操作，这里也就不用重新加锁，就没有加锁、解锁的开销了。</p>
<p>当第二个线程过来，两个线程产生了竞争，就会从偏向锁升级成自旋锁。自旋锁竞争状态中，没有获得锁的线程，就会不断自旋判断自己是否拿到了锁，没有拿到锁就会忙等。</p>
<h2 id="偏向锁-x2F-轻量级锁-x2F-重量级锁"><a href="#偏向锁-x2F-轻量级锁-x2F-重量级锁" class="headerlink" title="偏向锁&#x2F;轻量级锁&#x2F;重量级锁"></a>偏向锁&#x2F;轻量级锁&#x2F;重量级锁</h2><p>这三种锁是指锁的状态，并且是针对Synchronized。这三种锁的状态是通过对象监视器在对象头中的字段mark word来表明的。</p>
<ul>
<li>偏向锁是指一段同步代码一直被一个线程所访问，那么该线程会自动获取锁。降低获取锁的代价。也就是说一个对象在被初始化后，如果还没有任何线程来获取它的锁时，它就是可偏向的，当有第一个线程来访问它尝试获取锁的时候，它就记录下来这个线程，如果后面尝试获取锁的线程正是这个偏向锁的拥有者，就可以直接获取锁，开销很小。</li>
<li>轻量级锁是指当锁是偏向锁的时候，被另一个线程所访问，偏向锁就会升级为轻量级锁，其他线程会通过自旋的形式尝试获取锁，不会阻塞，提高性能。</li>
<li>重量级锁是指当锁为轻量级锁的时候，另一个线程虽然是自旋，但自旋不会一直持续下去，当自旋一定次数的时候，还没有获取到锁，就会进入阻塞，该锁膨胀为重量级锁。重量级锁会让其他申请的线程进入阻塞，性能降低(因为唤醒该线程，需要操作系统的帮忙，也就是涉及到用户态和内核态的切换，转换状态比较耗时，所以重量级锁开销很大)。</li>
</ul>
<p>总共有4种锁状态，级别由低到高依次为：<strong>无锁状态</strong>、<strong>偏向锁状态</strong>、<strong>轻量级锁状态</strong>、<strong>重量级锁状态</strong>。这几个状态会随着竞争情况逐渐升级。</p>
<p>注意：锁可以升级但不能降级。</p>
<p><img src="http://qiniu.c77544s.top/picgo/202210121457818.png" alt="img"></p>
<h2 id="Java-对象头和-Monitor"><a href="#Java-对象头和-Monitor" class="headerlink" title="Java 对象头和 Monitor"></a>Java 对象头和 Monitor</h2><p>在使用 synchronized 来同步代码块的时候，经编译后，会在代码块的起始位置插入 <strong>monitorenter指令</strong>，在结束或异常处插入 <strong>monitorexit指令。</strong>当执行到 monitorenter 指令时，将会尝试获取对象所对应的 **monitor **的所有权，即尝试获得对象的锁。而 synchronized 用的锁是存放在 <strong>Java对象头</strong> 中的。</p>
<p>所以引出了两个关键词：“Java 对象头” 和 “Monitor”。</p>
<h4 id="1、Java-对象头"><a href="#1、Java-对象头" class="headerlink" title="1、Java 对象头"></a>1、Java 对象头</h4><p>我们以 Hotspot 虚拟机为例，Hotspot 的对象头主要包括两部分数据：Mark Word（标记字段）、Klass Pointer（类型指针）。</p>
<p><strong>Mark Word</strong>：默认存储对象的 HashCode，分代年龄和锁标志位信息。这些信息都是与对象自身定义无关的数据，所以 Mark Word 被设计成一个非固定的数据结构以便在极小的空间内存存储尽量多的数据。它会根据对象的状态复用自己的存储空间，也就是说在运行期间 Mark Word 里存储的数据会随着锁标志位的变化而变化。</p>
<p><strong>Klass Point</strong>：对象指向它的类元数据的指针，虚拟机通过这个指针来确定这个对象是哪个类的实例。</p>
<h4 id="2、Monitor"><a href="#2、Monitor" class="headerlink" title="2、Monitor"></a>2、Monitor</h4><p>Monitor 可以理解为一个同步工具或一种同步机制，通常被描述为一个对象。每一个 Java 对象就有一把看不见的锁，称为内部锁或者 Monitor 锁。</p>
<p>Monitor 是线程私有的数据结构，每一个线程都有一个可用 monitor record 列表，同时还有一个全局的可用列表。每一个被锁住的对象都会和一个 monitor 关联，同时 monitor 中有一个 Owner 字段存放拥有该锁的线程的唯一标识，表示该锁被这个线程占用。</p>
<h2 id="3种锁对比"><a href="#3种锁对比" class="headerlink" title="3种锁对比"></a>3种锁对比</h2><p>偏向锁通过对比 Mark Word 解决加锁问题，避免执行CAS操作。</p>
<p>轻量级锁是通过用 CAS 操作和自旋来解决加锁问题，避免线程阻塞和唤醒而影响性能。</p>
<p>重量级锁是将除了拥有锁的线程以外的线程都阻塞。</p>
<p>转自：<a target="_blank" rel="noopener" href="https://www.cnblogs.com/shuiyj/p/13121393.html">图解 synchronized 的锁升级机制</a>、<a target="_blank" rel="noopener" href="https://www.jianshu.com/p/d61f294ac1a6">synchronized四种锁状态的升级</a></p>

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        <ol class="toc"><li class="toc-item toc-level-2"><a class="toc-link" href="#%E5%81%8F%E5%90%91%E9%94%81-x2F-%E8%BD%BB%E9%87%8F%E7%BA%A7%E9%94%81-x2F-%E9%87%8D%E9%87%8F%E7%BA%A7%E9%94%81"><span class="toc-text">偏向锁&#x2F;轻量级锁&#x2F;重量级锁</span></a></li><li class="toc-item toc-level-2"><a class="toc-link" href="#Java-%E5%AF%B9%E8%B1%A1%E5%A4%B4%E5%92%8C-Monitor"><span class="toc-text">Java 对象头和 Monitor</span></a><ol class="toc-child"><li class="toc-item toc-level-4"><a class="toc-link" href="#1%E3%80%81Java-%E5%AF%B9%E8%B1%A1%E5%A4%B4"><span class="toc-text">1、Java 对象头</span></a></li><li class="toc-item toc-level-4"><a class="toc-link" href="#2%E3%80%81Monitor"><span class="toc-text">2、Monitor</span></a></li></ol></li></ol></li><li class="toc-item toc-level-2"><a class="toc-link" href="#3%E7%A7%8D%E9%94%81%E5%AF%B9%E6%AF%94"><span class="toc-text">3种锁对比</span></a></li></ol>
      
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                });
              } else {
                isMatch = false;
              }
              // 0x05. show search results
              if (isMatch) {
                str += "<li><a href='" + data_url + "' class='search-result-title'>" + orig_data_title + "</a>";
                var content = orig_data_content;
                if (first_occur >= 0) {
                  // cut out 100 characters
                  var start = first_occur - 20;
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                  if (start < 0) {
                    start = 0;
                  }

                  if (start == 0) {
                    end = 100;
                  }

                  if (end > content.length) {
                    end = content.length;
                  }

                  var match_content = content.substr(start, end);

                  // highlight all keywords
                  keywords.forEach(function (keyword) {
                    var regS = new RegExp(keyword, "gi");
                    match_content = match_content.replace(regS, "<span class=\"search-keyword\">" + keyword + "</span>");
                  });

                  str += "<p class=\"search-result-abstract\">" + match_content + "...</p>"
                }
                str += "</li>";
              }
            });
            str += "</ul>";
            if (str.indexOf('<li>') === -1) {
              return $resultContent.innerHTML = "<ul><span class='local-search-empty'>No result<span></ul>";
            }
            $resultContent.innerHTML = str;
          });
        },
        error: function(xhr, status, error) {
          $resultContent.innerHTML = ""
          if (xhr.status === 404) {
            $resultContent.innerHTML = "<ul><span class='local-search-empty'>The search.xml file was not found, please refer to：<a href='https://github.com/zchengsite/hexo-theme-oranges#configuration' target='_black'>configuration</a><span></ul>";
          } else {
            $resultContent.innerHTML = "<ul><span class='local-search-empty'>The request failed, Try to refresh the page or try again later.<span></ul>";
          }
        }
      });
      $(document).on('click', '#search-close-icon', function() {
        $('#search-input').val('');
        $('#search-result').html('');
      });
    }

    var getSearchFile = function() {
        var path = "/search.xml";
        searchFunc(path, 'search-input', 'search-result');
    }
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